1. Field of the Invention
The present invention relates to a vehicle communication system for performing data communication between respective ECUs mounted on a vehicle.
2. Description of the Related Art
In recent years, as electronic instrumentation mounted on a vehicle has been increased, a large number of ECUs (electronic control units) have been provided in the vehicle. Moreover, in order for the ECUs to perform data communication to each other, communication lines for data transmission are cabled between the respective ECUs, and junction connectors (hereinafter, referred to as “J/Cs”) for branching the communication lines are provided.
FIG. 1 is an explanatory view showing a construction of a conventional vehicle communication system. As shown in this drawing, in the vehicle communication system, a J/C 103 is coupled to an ECU 101 serving as a starting point. A communication line branches into two lines through the J/C 103, and the branch lines are coupled to a J/C 104 and a J/C 105, respectively.
Six ECUs 102 are coupled to each of the J/C 104 and the J/C 105, and moreover, junction points in the J/C 104 and junction points in the J/C 105 are grounded through termination resistors RT, respectively. Furthermore, a length of the communication line that couples the ECU 101 serving as the starting point and the J/C 103 is set at 4 m, lengths of the communication lines between the J/C 103 and the J/C 104 and between the J/C 103 and the J/C 105 are set at 4.5 m, and lengths between the J/C 104 and the respective ECUs 102 and between the J/C 105 and the respective ECUs 102 are set at 2 m. In the drawing, Tx represents transmission and Rx represents reception.
In the vehicle communication system constructed as described above, in the case where signals are transmitted from the ECU 101 serving as the starting point to the respective ECUs 102, so-called ringing in which a signal waveform largely oscillates in the vertical direction sometimes occurs when the signals are switched from an ON state (dominant) to an OFF state (recessive).
FIG. 2 is a characteristic view showing a signal waveform generated in the communication line connected to the ECU 101 when the signal is transmitted from the ECU 101 of FIG. 1. In FIG. 2, when an output of the transmission signal is switched from the OFF state to the ON state at time t101, such an output signal of the ECU 101 reaches a predetermined voltage level, and a desired signal is transmitted to the other ECU 102.
Moreover, when the output of the transmission signal is switched from the ON state to the OFF state at time t102, though the output is turned off, a signal waveform thereof largely oscillates in the vertical direction owing to the ringing, and the oscillation is left until time t103 when the output is turned on the next time.
When the ringing as described above occurs, an error occurs in the communication signals between the ECU 101 and the respective ECUs 102, causing a problem that communication accuracy is lowered. Moreover, the ringing does not always occur. Characteristic impedance is changed depending on various conditions including the lengths of the communication lines connecting the ECUs and J/Cs to each other, sizes of the termination resistors RT coupled to the J/Cs 104 and 105, and the number of ECUs 102 coupled to the J/Cs 104 and 105. From the above, it is possible that the ringing occurs to a large extent and that the ringing does not occur at all depending on the magnitude of the characteristic impedance.
Hence, conditions where the ringing does not occur is estimated to some extent, and positions of the ECUs 101 and 102 and the J/Cs 103 to 105, the lengths of the communication lines, and the like in the vehicle are determined, thus making it possible to prevent the occurrence of the ringing. However, actually, the ringing sometimes occurs because the lengths of the communication lines are varied for each vehicle on which the ECUs, J/Cs, and the communication lines are mounted, other ECUs are additionally provided, and so on, varying the characteristic impedance.
In this connection, as a method of surely preventing the occurrence of the ringing, for example, as described in Japanese Patent Laid-Open Publication No. 2000-151153, there has been proposed a method of restricting the variations of the characteristic impedance by individually connecting drivers to respective bus lines in order to drive the bus lines.